U.S. patent number 4,048,814 [Application Number 05/675,064] was granted by the patent office on 1977-09-20 for refrigerating plant using helium as a refrigerant.
This patent grant is currently assigned to Sulzer Brothers Ltd.. Invention is credited to Hans Quack.
United States Patent |
4,048,814 |
Quack |
September 20, 1977 |
Refrigerating plant using helium as a refrigerant
Abstract
The plant includes a precooling stage, a Joule/Thomson stage and
a cryogenic load. The Joule/Thomson stage includes a pair of heat
exchangers in which the helium emerging from the precooling stage
is cooled by the return flow of helium. This stage also includes an
expansion turbine between the heat exchangers to expand the high
pressure helium to an intermediate pressure and an expansion
element in which the helium is expanded to the liquefaction
pressure. This expansion element is located upstream or downstream
of the cryogenic load or between two parts of the cryogenic
load.
Inventors: |
Quack; Hans (Pfaffikon,
CH) |
Assignee: |
Sulzer Brothers Ltd.
(Winterthur, CH)
|
Family
ID: |
4282358 |
Appl.
No.: |
05/675,064 |
Filed: |
April 8, 1976 |
Foreign Application Priority Data
|
|
|
|
|
Apr 15, 1975 [CH] |
|
|
4777/75 |
|
Current U.S.
Class: |
62/335; 505/895;
62/51.2 |
Current CPC
Class: |
F25J
1/0276 (20130101); F25B 9/10 (20130101); Y10S
505/895 (20130101); F25B 11/02 (20130101); F25B
9/002 (20130101) |
Current International
Class: |
F25B
9/10 (20060101); F25J 1/00 (20060101); F25B
9/00 (20060101); F25B 11/02 (20060101); F25B
007/00 () |
Field of
Search: |
;62/62,79,335,514R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Capossela; Ronald C.
Attorney, Agent or Firm: Kenyon & Kenyon, Reilly, Carr
& Chapin
Claims
What is claimed is:
1. A refrigerating plant having a helium circuit in which helium
circulates as a refrigerant, said plant comprising
a precooling stage to precool a flow of helium to a temperature
below the inversion temperature thereof with the performance of
work, said stage including means for compressing the flow of helium
and means for cooling the compressed flow of helium;
a Joule/Thomson stage including a pair of heat exchangers
interposed in said helium circuit to place a flow of helium from
said precooling stage in heat exchange relation with a flow of
helium to said precooling stage to cool the flow of helium from
said precooling stage to the liquefaction temperature thereof, an
expansion turbine between said pair of heat exchangers in the flow
of helium from said precooling stage for expanding the flow of
helium to an intermediate pressure, and an expansion means in the
flow of helium between the exit of the heat exchanger of said pair
of heat exchangers downstream of said expansion turbine in the flow
of helium from said precooling state and the entry to said heat
exchanger in the flow of helium to said precooling stage for
expanding the flow of helium to a liquefaction pressure; and
a cryogenic load for supplying heat to the flow of helium, said
load being disposed between said exit and said entry of said heat
exchanger in the flow of helium.
2. A refrigerating plant as set forth in claim 1 wherein said means
for cooling in said precooling stage is a means for expanding the
flow of helium with the performance of work.
3. A refrigerating plant as set forth in claim 1 wherein said
expansion means is upstream of said cryogenic load relative to the
flow of helium.
4. A refrigerating plant as set forth in claim 1 wherein said
expansion means is downstream of said cryogenic load relative to
the flow of helium.
5. A refrigerating plant as set forth in claim 1 wherein said
cryogenic load includes two parts for individually supplying heat
to the flow of helium and said expansion means is disposed between
said two parts relative to the flow of helium.
6. A refrigerating plant as set forth in claim 1 wherein said
expansion means is a throttle valve.
7. A refrigerating plant as set forth in claim 1 wherein said
expansion means is an expansion turbine.
Description
This invention relates to a refrigerating plant using helium as a
refrigerant.
Refrigerating plants have been known which use helium as a
refrigerant within a refrigerant circuit. For example, one such
plant has been known which includes a precooling stage in which
helium is compressed and is cooled by heat exchange and also,
preferably, by expansion with the performance of work, a
Joule/Thomson stage in which the helium cooled to a precool
temperature below its inversion temperature is cooled to its
liquefaction temperature by heat exchange and expansion with the
performance of work, and a cryogenic load which supplies heat to
the helium by heat exchange.
In the precooling stage of plants of this kind a compressor
generally draws in helium gas at atmospheric pressure and ambient
temperature and compresses the gas to a higher pressure. The gas is
then cooled to a precool temperature below the inversion
temperature by heat exchange in a number of countercurrent heat
exchangers with low-pressure helium gas flowing back to the
compressor from a helium reservoir and by expansion with the
performance of work.
In simpler plants, the Joule/Thomson stage conventionally consists
of a heat exchanger and a throttle valve in which the helium gas is
expanded to liquefaction pressure. In such cases, the helium gas is
cooled further in the heat exchanger by heat exchange with
low-pressure helium gas, from the precool temperature to an
enthalpy less than the enthalpy in the case of saturated vapor at
one atmosphere pressure. In this way, partially liquid helium forms
on throttling.
As is known, the refrigerating capacity of a refrigerating plant of
the above type is determined by that quantity of heat which can be
supplied by the cryogenic load to the liquid helium in order to
vaporize the amount of liquid helium forming on throttling.
The energy balance of the Joule/Thomson stage with throttled
expansion shows that the refrigerating capacity is determined by
the product of the quantitative flow in the Joule/Thomson stage and
the enthalpy difference of the two helium flows entering and
leaving the heat exchanger at the hot end of the heat exchanger. To
achieve a large enthalpy difference in order to obtain the maximum
refrigerating capacity, the tendency is to keep the temperature
difference at the hot end of the heat exchanger as small as
possible, e.g. of the order of 0.2.degree. K.
The plant refrigerating capacity, however, can be improved if the
throttle valve in the Joule/Thomson stage is replaced by an
expansion machine in which the high-pressure helium gas is expanded
with the performance of work. The increase in the refrigerating
capacity then corresponds to the energy dissipated by the expansion
machine.
Low temperature refrigerating plants have already been proposed
with a piston expansion machine in the Joule/Thomson stage. In such
cases the helium emerges from the machine in the form of a
gas-liquid two-phase flow. However, piston expansion machines are
very susceptible to trouble because of inevitable mechanical
friction. Consequently, one might therefore consider the use of
expansion turbines instead.
It is well known that the efficiency of expansion turbines depends
greatly on the size of the volume flow, the efficiency of an
expansion turbine being in direct proportion to the volume flow.
However, the specific volume of the high-pressure helium flow is
very small on leaving the heat exchanger of the Joule/Thomson stage
in the hitherto conventional constructions thereof. Thus,
relatively large quantitative flows would therefore be necessary to
obtain the maximum efficiency of the expansion turbine.
Accordingly, it is an object of the invention to provide a
refrigerating plant with an improved Joule/Thomson stage.
It is another object of the invention to increase the refrigerating
capacity of refrigerating plants using a Joule/Thomson stage.
It is another object of the invention to increase the efficiency of
an expansion turbine in a Joule/Thomson stage of a refrigerating
plant.
Briefly, the invention is directed to a refrigerating plant having
a helium circuit in which helium circulates as a refrigerant and
particularly to a plant comprising a precooling stage, a
Joule/Thomson stage and a cryogenic load.
The precooling stage functions to cool a flow of helium to a
temperature below the inversion temperature of the helium with the
performance of work. To this end, the stage includes a means for
compressing the flow of helium and means for cooling the compressed
flow of helium.
The Joule/Thomson stage includes a pair of heat exchangers which
are interposed in the helium circuit to place a flow of helium from
the precooling stage in heat exchange with a flow of helium to the
precooling stage to cool the flow of helium to the liquefaction
temperature. In addition, the Joule/Thomson stage includes an
expansion turbine between the heat exchangers for expanding the
flow of helium to an intermediate pressure and an expansion means
between an exit of the heat exchanger downstream of the expansion
turbine, i.e. the low pressure turbine, and an entry to the heat
exchanger for expanding the flow of helium to a liquefaction
pressure.
The cryogenic load supplies heat to the flow of helium and is
disposed between the exit and entry of the low pressure exchanger
in the flow of helium.
The main advantage of the invention over the Joule/Thomson stage
suggested above wherein use is made of a single heat exchanger
followed by an expansion turbine in which the helium is expanded
from high-pressure to liquefaction pressure is that the volume flow
in the expansion turbine is increased because of the higher entry
temperature of the high-pressure helium flow to the expansion
turbine. This is due to the use of two heat exchangers and the
following expansion to liquefaction pressure, the effect of which
is to improve efficiency of the expansion turbine and hence achieve
greater refrigerating capacity.
The provision of an expansion means after the second heat exchanger
has the effect that the high-pressure helium flow can be expanded
in the expansion turbine to an intermediate pressure higher than
the liquefaction pressure of helium. The result of this
intermediate pressure is that a positive temperature difference
between the two helium flows in heat exchange with one another can
be maintained in the second heat exchanger. Thus, heat can be
transferred from the helium gas expanded with the performance of
work to the helium vaporized in the cryogenic load.
Although a cryogenic load is frequently located at some distance
from the remainder of the refrigerating plant, a pressure drop
(pressure difference between the intermediate pressure and the
liquefaction pressure) is available for the pipeline to the
cryogenic load with the construction of the invention. Thus, the
final expansion would take place at least partly in the pipeline
itself and, in this special case, the pipeline itself can be
regarded as the expansion means.
On the other hand, with some cryogenic loads, it is desirable that
helium should flow through the cryogenic load at a supercritical
pressure to give better heat transfer. This flow is conventionally
supercooled in a reservoir with liquefied helium. In that case,
expansion to the low pressure takes place in the expansion means so
that liquid helium is not produced until the intermediate-pressure
gas has passed through the load.
If the arrangement described hereinbefore, i.e. with a single heat
exchanger in the Joule/Thomson stage and subsequent expansion of
the high-pressure helium flow, were to be used, the result would be
a reduced refrigerating capacity since in that case the
high-pressure gas would also have to be expanded to an intermediate
pressure in order to supply the pressure drop in the pipeline to
the cryogenic load or allow cooling in the supercritical range of
the helium.
Apart from the case in which the pressure gradient available from
the intermediate pressure to the liquefaction pressure is
completely used up in the pipeline, the expansion means may be a
throttle valve or may be another expansion turbine. This allows the
refrigerating capacity to be increased still further.
These and other objects and advantages of the invention will become
more apparent from the following detailed description and appended
claims taken in conjunction with the accompanying drawings in
which:
FIG. 1 illustrates a flow diagram of a previously proprosed helium
refrigerating plant with a precooling stage and a Joule/Thomson
stage;
FIG. 2 illustrates a Joule/Thomson stage of another previously
proposed refrigerating plant with a possible modification;
FIG. 3 illustrates a Joule/Thomson stage of a refrigerating plant
embodying the invention;
FIG. 4 illustrates a modified Joule/Thomson stage of another
refrigerating plant embodying the invention; and
FIG. 5 illustrates a modified arrangement having an expansion
turbine as an expansion means.
Referring to FIG. 1, a previously proposed refrigerating plant
using helium as a refrigerant within a closed circuit comprises a
precooling stage I consisting of a compressor 1 with a cooler 2 for
dissipating compression heat, heat-exchangers 3 to 6 and expansion
turbines 7 and 8. When in use, a compressed helium gas is cooled to
a precool temperature below the inversion temperature in the
precooling stage by heat-exchange or by expanding a helium branch
flow with the performance of work. Proposals have also been made to
obtain cooling by means of external refrigerants, e.g. nitrogen and
hydrogen, via heat exchange, instead of cooling by means of
expansion turbines.
The plant also comprises a Joule/Thomson stage IIa consisting in
this case of a single-unit heat exchanger 9 and a throttle valve 10
in which the helium gas is expanded from high pressure to
liquefaction pressure of about 1 atmosphere, and is at the same
time partially liquified.
The liquified helium is collected in a reservoir 11 connected to
the Joule/Thomson stage and a cryogenic load 12, for example the
coil of a superconductive magnet which, is diagrammatically
illustrated as being located in the reservoir.
Referring to FIG. 2, wherein like reference characters indicate
like parts as above described, another previously proposed
refrigerating plant uses a precooling stage of similar construction
to that of FIG. 1 and a Joule/Thomson stage IIb which has a helium
reservoir with a cryogenic load located therein. However, unlike
the plant shown in FIG. 1, a piston expansion machine 13 is
disposed after the heat exchanger 9 instead of a throttle valve.
Also, in this machine 13, the helium gas is expanded from high
pressure to liquefaction pressure with the performance of work and
is partially liquefied in these conditions.
The broken lines in FIG. 2 show the possible replacement of the
piston expansion machine 13 by an expansion turbine 14. It is
believed that this construction of the Joule/Thomson stage has not
previously been proposed but is being mentioned to provide a better
explanation of the advantages of the invention and will be
discussed in a numerical example hereinafter.
Referring to FIG. 3, wherein like reference characters indicate
like parts as above, the refrigerating plant according to the
invention uses a precooling stage of similar construction to that
shown in FIG. 1. However, by contrast with FIGS. 1 and 2, the
Joule/Thomson stage has two separate heat exchangers 15a and 15b in
order to effect a heat exchange in the countercurrent flows of the
helium. In addition, an expansion turbine 16 is disposed in the
high-pressure helium flow between the two heat exchangers and an
expansion means in the form of a throttle valve 17 is provided
between the second heat exchanger 15b and the helium reservoir
11.
As shown, the expansion turbine 16 is located via pipelines h, i,
in the flow of helium passing from the precooling stage via a pipe
while the throttle valve 17 is located in a pipeline j downstream
of the turbine 16 between the exit of the low pressure heat
exchanger 15b and the reservoir 11.
Referring to FIG. 4, wherein like reference characters indicate
like parts as above, a modified refrigerating plant of the
invention may have a cryogenic load which is cooled in the
supercritical range. The helium flow which is expanded in the
Joule/Thomson stage IId to an intermediate pressure with the
performance of work and which is further cooled in the heat
exchanger 15b is further cooled by heat exchange with liquefied
helium upon passing through a coil 18 in the reservoir. The helium
then flows through the cryogenic load 20, e.g. a magnet coil
constructed as a tubular conductor, and is then expanded to
liquefaction pressure in the throttle valve 21 and fed into the
helium reservoir 19.
As already explained hereinbefore and as will be apparent from the
following numerical example, a refrigerating plant provided with a
Joule/Thomson stage according to the invention can give a higher
refrigerating capacity than the constructions shown in FIG. 1 and
in FIG. 2 with the expansion turbine 14 under otherwise identical
conditions, such as identical precooling temperature and identical
mass flow. This effect is achieved by using at least one expansion
turbine which is extremely reliable in operation unlike a piston
expansion machine.
The important variables for the numerical example given in the
following Table are the pressures in atmospheres, temperatures in
.degree.K. and enthalpies (J/g) at the places marked a to k in
FIGS. 1 to 3.
______________________________________ Pressure Temperature
Enthalpy h Place p (atm) T (.degree. K) (J/g)
______________________________________ a 1 13.8 85.26 b 16 14.0
73.06 c 1 4.224 30.13 d 16 4.655 17.93 e 1 4.224 17.93 f 1 4.224
11.23 g 1 5.29 38.06 h 16 6.826 25.86 i 3 5.44 18.85 j 10 4.374
10.92 k 1 4.224 10.92 ______________________________________
The following values of the specific refrigerating capacity are
obtained with the individual embodiments.
Refrigerating plants according to
Fig. 1: h.sub.c - h.sub.e = 12.2 J/g
Fig. 2: h.sub.c - h.sub.f = 18.9 J/g
Fig. 3: h.sub.c - h.sub.k = 19.21 J/g
All three embodiments are based on the same precool temperature
T.sub.b = 14.0.degree. K. and the same exit temperature of the
low-pressure helium flow T.sub.a = 13.8.degree. K. and the same
mass flow.
______________________________________ Comparison of the important
variables of the expansion turbines in Figs. 2 and 3. Fig. 2 Fig. 3
______________________________________ Inlet pressure 16 atm 16 atm
End pressure 1 atm 3 atm Inlet temperature 4.655.degree. K 6.826
Exit temperature 4.224.degree. K 5.440 Isentropic drop 10.31 J/g
10.4 J/g Specific volume 6.386 cm 3/g 7.328 cm 3/g at inlet
Efficiency 0.65 0.674 Dissipated energy 6.70 J/g 7.01 J/g
______________________________________
In the second case, the expansion turbine dissipates more energy
for two reasons: First, because the isentropic drop is already
greater (10.4 as against 10.31) and, second, because the turbine
has a better efficiency (0.674 as against 0.65) because of the
greater specific volume of the gas.
It should be noted that the extent to which the efficiency of the
expansion turbine improves with increasing volume flow depends on
the type of construction and overall size of the expansion
turbine.
The difference in efficiency from 0.674 to 0.650 in the numerical
example is typical of relatively small expansion turbines for a
throughput volume of the order of 500-1000 cubic centimeters per
second (cm.sup.3 /s) for a specific inlet volume difference of
15%.
Various modifications may be made to the plants shown in FIGS. 3
and 4. For example, the throttle valves 17 and 21 may be replaced
by expansion turbines 22 (FIG. 5). In the construction shown in
FIG. 4, the cryogenic load 20 may be divided into two parts with
the throttle valve 21 between them.
* * * * *